1. Field Of the Invention
This invention relates to a system for locating the source of an electromagnetic wave emission, and in particular to a system for locating radar emissions.
2. Description of Related Art
An extremely important problem encountered in many tactical military engagements is the need to locate hostile radar, and in particular the need to provide autonomous emitter location from a single airborne observation platform. Conventional passive emitter location techniques do not adequately address this problem.
Traditional emitter location techniques are generally based on the passive interception and processing of the hostile radar to yield a bearing or bearing-rate (or, in the case of phase interferometry, a phase or phase-rate) to the emitter, which can subsequently be used to determine the emitter's location by triangulation. However, it is well known that these triangulation techniques suffer from an acute sensitivity to angle measurements, and therefore require extremely large antenna apertures. In addition, conventional triangulation requires many noncoincident measurements, resulting in an unacceptably long convergence time to achieve acceptable ranging performance.
In order to decrease the convergence time of a bearing-only passive emitter location system, it has previously been proposed to apply a technique known as single-measurement delayed initialization (SMDI) to a Cartesian formulation extended Kalman filter (EKF). The basic premise of extended Kalman filtering is to filter an a posteriori state error covariance update equation about a filtered state estimate, rather than about an actual measurement. The initial estimate is obtained, using SMDI, by first multiplying the a priori probability density function (PDF), which represents the uncertainty concerning the emitter's location, by a first-measurement PDF in order to narrow the uncertainty before initialization. The SMDI technique therefore delays the initialization of the EKF until the single-measurement a posteriori PDF (SMAP-PDF) is obtained, after which the EKF is initialized with the mean of the SMAP-PDF and the state error covariance matrix derived from the SMAP-PDF.
Initially, there is an extremely large uncertainty concerning the imager's location, which one could take to represent the 1-.sigma. circular error ellipse of a jointly gaussian PDF. However, after a single measurement has been taken, the uncertainty region is reshaped as a result of superimposing the antenna pattern onto the a priori PDF. The resulting distribution is the intersection of the a priori CEP or circular error PDF with the antenna pattern. Assuming that the measurement is statistically independent of the a priori state estimate, the resulting single measurement a posteriori PDF is the product of the a priori PDF and the single-measurement PDF, which is essentially just the normalized antenna pattern.
In order to simplify the SMDI initialization, information is extracted from the SMAP-PDF by rotating the ownship-centered coordinate system such that one of the axes is aligned with the first measured bearing, setting the SMDI EKF estimate of position (x.sub.0, y.sub.0) equal to (R.sub.0, 0), where R.sub.0 is the original a priori mean range estimate, and by setting the SMDI EKF covariance matrix equal to a diagonal matrix with (.sigma..sub.x, .sigma..sub.y)=(.sigma..sub.r, R.sigma..sub..THETA.), where .sigma..sub.x and .sigma..sub.y are the x and y standard deviations associated with SMDI estimate (x.sub.0, y.sub.0)=(R.sub.0, 0), .sigma..sub..THETA. is the 1-.sigma. antenna beam width, and .sigma..sub.R is the standard deviation associated with the original a priori mean range estimate. This is equivalent to assuming that the target lies along the first measured bearing, with the associated standard deviations, and that the along-axis and cross-axis are uncorrelated.
While this technique greatly reduces the convergence time relative to the same filter without SMDI, the bearings-only system still inherently involves triangulation and, therefore, an acute sensitivity to angle measurements as well as a relatively large antenna aperture are still required to achieve acceptable range performance.
An alternative technique is to measure the Doppler induced frequency shift imposed on the radar's carrier frequency due to relative changes in relative position between the emitter and sensor. This technique eliminates the need for large antennas, but still requires a relatively long time to converge to an acceptable emitter location accuracy, and requires rapid maneuvering of the antenna platform or aircraft in order to collect the induced frequency shifts necessary for acceptable ranging performance.
Recently, there have been attempts to combine bearing and frequency measurements to achieve maximum passive emitter location performance, for example as described in K. Becker, "An Efficient Method of Passive Emitter Location", in IEEE Transactions on Aerospace and Electronic Systems, Vol. 28, No. 4, October 1992. These techniques are based on the simultaneous measurement of bearing and frequency, and require the measurement errors to be independent. However, the latter requirement can only be justified if two separate sensors are used which are also significantly separated in space. Thus, these beating-frequency (BF) techniques are not suited to the aforementioned single airborne observer platform case.